In Reddit, users can create lists of subs, called "multireddits". And you can browse the content of all those subs in a multireddit as if it was a single community. You can also share your multireddits with other people.

Reddit itself implemented the idea and never touched it again, but it be amazing in the federation. For example, someone who's interested in cooking could create the following ~~multireddit~~ multicomm:

• !foodporn@lemmy.world
• !food@beehaw.org
• !food@lemmy.ml
• !culinary@lemmy.world
• !shittyfoodporn@lemmy.ca
• !recipes@lemmy.ml

That increases discoverability of the communities across the Lemmyverse (as people share their multicomms), and also makes it easier to handle redundant communities across instances. Because of that, I feel like the concept would be right at home in Lemmy.